1. Field of the Invention
This invention relates to an electrode for an electric discharge machining apparatus suitable for the chipless cutting of openings in structural members, in particular, for hot-tapping pipes.
2. Description of the Prior Art
Electric discharge machining is a high precision process used in making machine dyes and other high precision parts. To provide the accuracy required in producing such high precision parts, electric discharge machining removes material at a slow rate. In addition, unlike mechanical cutters which produce large, sharp chips, sharp edges on the structural member being cut, and which apply large torques to the structural member during the cutting process, electric discharge machining produces very small round chips, smooth edges, and applies no torque to the structural member being cut.
As a result, electric discharge machining is of particular interest in applications requiring the hot-tapping of pipes, such as in-situ gas, oil or water pipelines. In such applications, it is desired to minimize the size of particles, generated in the cutting operation, which may fall into the pipe and become lodged in valve and regulator mechanisms, damaging them in the process. Smooth edges on the opening in the pipe are also important to prevent damage to rubber stoppers and similar devices inserted into the opening. It is also important that torque not be applied to the pipe during the cutting process to avoid undue stress upon the pipe.
In the electric discharge machining process, an electrode is brought in close proximity to an electrically conducting work piece creating a gap between the work piece and the electrode. The gap is gently flushed with a dielectric fluid as a pulsed DC voltage is applied across the gap. The dielectric fluid is ionized at a localized spot as a large current (tens of amperes) flows across the gap, vaporizing a portion of the work piece.
U.S. Pat. No. 3,873,800 discloses an electric discharge machining apparatus for enlarging elongated openings or bores in an electrically conductive work piece having an electrode which is rectangular in an end cross section. To produce an axial bore having an irregular end cross section, the electrode is oscillated in a to-and-fro swinging path of travel.
Due to the precision required by most electric discharge machining applications, the cutting process generally requires a substantial amount of time to complete. One method of increasing the speed of electric discharge machining is taught by U.S. Pat. No. 4,891,162 in which an electrical discharge machining fluid, that is, a dielectric fluid, is used, which permits operation at an increased machining speed and results in clean machined surfaces. However, the increases in cutting rates provided by such a dielectric fluid are not sufficient to satisfy the requirements of high productivity in hot-tapping processes.
Due to the generation of a spark during operation of an electric discharge machining apparatus, it is generally thought by those skilled in the art that electric discharge machining is not suitable for use in applications involving highly combustible materials. U.S. Pat. No. 4,536,633 teaches a dielectric fluid for electrical discharge machining having a high flash point and low combustibility which is not easily ionizable for reducing the combustion hazard.
In applications, such as natural gas pipelines, where the explosion hazard is great, the use of electric discharge machining in combination with known hot-tap technology as taught by U.S. Pat. No. 4,331,170, U.S. Pat. No. 4,332,272, and U.S. Pat. No. 4,541,447 has been shown to be an effective and practical method of cutting holes in pipes under pressure.
When hot-tapping pipelines in operation, it is important that the rate of cutting be sufficiently high to provide a high level of productivity. Thus, the cutting speed is a very practical consideration in selecting a cutting process and apparatus.
One of the key elements of an electric discharge machining apparatus is the electrode. It is known that a 6 inch diameter copper tube having a thickness of 0.125 inches when used as an electrode in an electric discharge machining apparatus requires an unacceptably long cutting time of 59 minutes, primarily because the current carrying capability of the electrode, and thus the material removal rate, decreases with electrode thickness. On the other hand, thin electrodes are reputed to vibrate and distort, requiring removal of more material than a thicker electrode, and thus requiring more rather than less time to cut through a structural member.